Polymer processing, characterisation and applications

Transcripts - Polymer processing, characterisation and applications

1.
Polymer Processing,
Characterisation and
Applications

2.
PLASTICS, ELASTOMERS
AND FIBERS

3.
Plastics
• Plastics and resins or Polymers are not synonymous
though they are in use interchangeably.
• Polymers or otherwise known as resins are the
products of polymerization
• Pure polymers or resins cannot be processed into end
products with required properties.
• Hence polymers or resins are to be admixed with
several ingredients and the resultant mix is called
plastics and the process of mixing is known as
compounding of plastics.
• Materials which can be deformed into desired shape
under the action of heat and/or pressure are known as
plastics.

4.
Types of Plastics
Thermo Plastics
• They soften on heating and
harden on cooling.
• They are made up of linear
or branched polymers
• Fusible and soluble
• Can be remoulded and
reused
• Reclamation and recycling
of waste is possible.
Thermosetting Plastics
• They harden on heating and
once hardened, it can not be
softened.
• Made up of cross-linked
polymers
• Infusible and insoluble
• Cannot be remoulded and
reused.
• Reclamation and recycling is
not possible

7.
2. PVC
• Vinyl cholride until recently was mainly produced from
acetylene
• Dry HCl and acetylene gases in equimolar proposition
is passed through multitubular reactor packed with
mercuric chloride catalyst supported on activated
carbon at 100 oC.
• When ethylene become abundant, it is chlorinated to
dichloroethane and it is cracked into Vinyl chloride and
HCl.
• Ethylene is also oxichlorinated to dichloroethane in the
presence of copper chloride catalyst followed by
cracking to vinyl chloride.
• PVC is mostly produced by emulsion and suspension
polymerisation of vinyl chloride in water using free
radical initiator.

9.
3. Polytetrafluoroethylene or Teflon
• Tetrafluoroethylene is produced first by
reacting chloroform with HF into
monochlorodifluoromethane followed by its
pyrolysis in the presence of Pt at 700 oC.
• CHCl3 + 2HF CHClF2 + 2HCl
• 2CHClF2 CF2=CF2 + 2HCl
• Teflon is produced by polymerising the
aqueous solution of tetrafluoroethylene using
free radical initiator.

15.
Elastomers
• Are long flexible chains with weak intermolecular
forces and occasional cross links.
• It can be deformed to large extent (Several
hundreds times)
• Once the deforming force is removed, regain
their original shape.
• At molecular level they have coil like structure
resembling that of steel spring.
• Upon being stretched, coil get unwounded and
straightened.
• Once the deforming force is removed, they regain
the coil like structure.

17.
Natural Rubber
• By making incisions on the barks of Rubber trees
(Havea brasiliness), rubber latex which is an
emulsion of 25-45% rubber in water along with
proteins oozes out.
• Latex is diluted so as to contain 15 – 20% rubber.
• Coagulation is done in tanks by adding 1 kg of
acetic acid or formic acid per 200 kg of rubber.
• Soft, white coagulum is washed with water and
dried.

18.
Natural rubber
• a) Crepe rubber
• Coagulum is bleached with sodium bisulphite.
• Bleached rubber is passed through creping machine
from which coagulum rolls out with irregular surface of
1 mm thickness. The sheet is then dried at 50 oC in air.
• b) Smoked rubber
• The bleached coagulum is rolled in to thicker sheets
having ribbed pattern which prevents them from
adhering on stalking and also increases the surface
area.
• The sheets are dried in smoke houses at 50 oC by
burning out wood or coconut shells.
• It is amber in colour..

23.
Non-Diene Elastomers
1. Butyl or Polyisobutylene Rubber
Produced by co-polymerizing isobutylene with 0.5
to 2% of isoprene by cationic polymerization..
Impermeable and extremely resistant to air
2. Polysiloxanes
Ex:Polydimethyl siloxane

33.
Transfer Moulding
• It is mainly developed to overcome the disadvantage of
compression moulding which is slow and poor heat
transmission which limits the products that can be
moulded.
• Charge is melted below the curing temperature in a
separate chamber and transferred into the closed and
hot mould.
• As the plastic enter the mould as melt, large and
intricate shapes can be filled unlike compression
moulding.
• Products have high density and mechanical strength.

34.
Transfer Moulding

35.
Extrusion Moulding
• Forcing the molten thermo plastic through the
die to get the product of uniform cross section
like pipe, rod, insulated wire etc.

36.
Extrusion Moulding

37.
Blow Moulding
• Mainly to produce hollow articles such as
containers.
• Techique was borrowed from glass industry
• Plastic is made into tube called parison by
either extrusion or injection and accordingly
known as extrusion blow moulding and
injection blow moulding respectively.

38.
Blow Moulding

39.
Thermoforming
• For making trays from plastic sheet.
• Vacuum or compressed air is used for forming.
• For deep moulds, plug assist is used.

40.
Thermoforming

41.
Calendering
• For the continuous formation of sheet or film
• Calender usually consists of four highly
polished rolls commonly arranged in ‘Z’, ‘I’ or
inverted ‘L’ shape
• The soft or dough like plastic mass is metered
between the hot rolls to give product of
uniform thickness.
• Embossing effect or surface design can be
produced using engraved calender roll.

42.
Calendering

43.
Spinning

44.
Melt Spinning
• It is used for polymers which are stable at
their melting point.
Ex:
• Polyethylene
• Poly propylene
• Nylon-6 and Nylon-6,6
• PET

45.
Melt Spinning

46.
Solution Spinning
• Polymers which are not thermally stable at
their melting point are processed by solution
spinning
• Dry Spinning
• Solvent is evoporated by passing hot gases.
• Ex: Cellulose acetate and acrylics

51.
Mastication and Mixing
• Process of breakdown of the molecular chains of
the rubber by shearing action is known as
mastication
• It makes the rubber
• - Soft
• - flows more readily
• - to form solution of very high concentration with
low viscosity
• - tacky (sticks to itself) so that articles of suitable
thickness can be made from layers of mastcated
rubber.

55.
Compounding Ingredients
• Peptizers
• added at the beginning of mastication
• Act chemically on the rubber and accelerate
the rate of breakdown of rubber chains and
increase the efficiency of mastication
• Ex: Zinc thiobenzoate, thio-β-naphthol
• Other ingredients are added after the
mastication yields rubber of desired plasticity.

57.
Curing agent
 Sulphur is the most commonly used for curing rubber
at temperatures above 140 oC for a minimum curing
time of 8 hrs with sulphur dose of 8-10 phr.
 Sulphur monochloride can bring curing at room
temperature.
 Peroxides, metal oxides, amines, amine derivatives and
oximes are other curing agents for selected rubbers.
 Selinium and tellurium can substitute sulphur as curing
agent partially or completely.
 High energy radiation can bring effective curing but not
developed into a commercial process

62.
Advantages of accelerated Sulphur Vulcanisation
Incorporation of 0.2 to 2 phr of accelerator
brings down the sulphur dose from 8-10 to
0.5-3 phr and curing time from several hours
(8 hr) to few minutes-an hour.
Low sulphur requirement of accelerated
sulphur vulcanisation technology has
eliminated bloom i.e migration of unreacted
sulphur to the surface of vulcanised rubber
and yielded vulcanised rubber of improved
physical properties and good heat resistant
and aging.

63.
Choice of Accelerators
• Choice is dictated by nature of rubber, design of the product
and processing conditions.
• With increase in curing time, initially there is a sharp increase
in modulus and after reaching maximum value either the
modulus remains same (Ex: SBR) or decreases, called
reversion in rubbers like Natural rubber.

64.
Choice of accelerators
• Scorching or premature vulcanisation during
compounding is undesirable and this problem
with ultra accelerator.
• In thick rubber products slow accelerators are
preferred. As rubber is poor conductor of heat
it takes more time the interiors of product to
get heated by the heat flowing from the
surface. Hence surface layers get overcured by
the time curing begins at the interiors.

65.
Choice of Accelerators
• For rubbers with limited unsaturation like
EPDM, butyl rubber, fast accelerators are to be
used at high curing temperatures.
• For butyl rubber showing reversion, duration
of curing and curing temperature are to be
carefully controlled.
• Ideal accelerator is the one which remains
stable duting compounding, storing and
processing of the mix but readily reacts at the
curing temperature.

66.
Accelerator Activators
• The effect of accelerators is enhanced by the
addition of specific additives known as accelerator
activators.
• They are mainly two component systems comprised
of metal oxide and a long chain fatty acid. Ex: Zinc
oxide and stearic acid

67.
Additives ….
• Activators must have good dispersability or
solubility in rubber.
• Retarders
To minimise the hazard of scorching, retarders are
added. Ex: Acids like Phthalic anhydride
• Antidegradants
To avoid degradation of rubber due to attack by
oxygen and ozone, antioxidants or antiozoonants
are added
1.Amine type Ex: β – Napthylamine
2.Phenol type Ex: styrenated phenols.

68.
Polymer Blends
• Mixing together of two or more polymers or
copolymers to homogeneous mass having
properties different from the constituents.
• Key for making polymer blends is the
compatibility between polymers.
• Use of compatibilizers can bring down the
phase separation in blends.

70.
Mechanical polyblends
• -made by melt blending of constituent
polymers
• For amorphous polymers blending
temperature must be above Tg of all the
constituting polymers and in the case of
semicrystalline polymers it must be well above
Tm.
• Is used for polymers which donot thermally
degrade.

71.
Chemical polyblends and
Mechano-chemical polyblends
• Chemical polyblend is given by chemically
linking polymers either in the axial or in cross
direction giving block copolymer or craft
copolymer structure respectively.
• Mechanical blends also undergo cross-linking
or terminal linking and been called as
mechano-chemical polyblends

72.
Solution-cast polyblends
• Polymers are dissolved in solvents and thus
lower the temperature and shear force
necessary to have uniform mixing
• But solvent must be completely removed after
the casting.
• But after the removal of solvent, it can leave
significant changes in the property of the
blend

73.
Latex Polyblends
• Most important technique commercially
• Polymers are made into suspension of
microspheres of specific size with the help of
stabilisers (Latex)
• When different latexes are blended, latex mixture
containing different polymers is obtained.
• When the latex mixture is coagulated, intimate
mixture of constituent polymers is obtained.

76.
Properties of Polyblends …
• If ∆Gm is positive over entire composition range at
given temperature, two polymers in the poly blend will
separate into pure phases of each polymer.
• For immiscible polyblend giving continuous phase
(Phase 1) and dispersed phase (Phase 2)
• P/P1 = (1+ AB φ2)/(1-Bψ φ2)
• Where φ2 is the concentration of the dispersed phase
• Value of A varies 0 to infinity depends on the shape
and orientation of dispersed phase
• Value of B depends on relative values of P1, P2 and A
• Ψ is a reduced concentration which is a function of
maximum packing fraction
• When A 0, dispersed phase is soft and
• A infinity, dispersed phase is hard.

78.
Practical aspects of Polymer blending ..
• Heterogeneous polyblends have application in
high impact plastics.
• Blending of brittle plastic with small amount of
rubber improves the impact resistance with
diminished modulus.
• By cross linking the rubber in the blend, resultant
product shows improved toughness, stiffness and
impact resistance by uniformly distributing
rubber in the blend and appropriate size of
dispersed rubber phase.
• Heterogeneous polyblends scatters the light
according to the size of dispersed phase. Larger
their size more will be the scattering and behave
as opaque.

81.
3.Elastomer-Plastomer blends
• General immiscibility of polymers is turned
into advantage in making rubber-toughened
plastics (Different from Thermoplastic
elastomers)
• Polyolefin thermoplastics like PE, PP are
blended with EPDM, NR
• By cross linking the eleastomeric component,
properties can be further improved and cross
linked elastomer-plastomer blends are known
as Thermo-plastic vulcanizates(TPVs)